1. Field of the Invention
This invention relates to apparatus for scribing grain-oriented electrical steel strip, and more particularly to apparatus for scribing grain-oriented electrical steel strip that improves the core loss thereof by forming linear impressions on the surface thereof using oppositely disposed metal dies.
2. Description of the Prior Art
One of the practices that has come to be adopted recently in the manufacturing of grain-oriented electrical steel strip is to form linear impressions on the surface thereof. The object of this practice is to reduce the domain size by forming linear impressions, thereby improving core loss and magnetic properties.
But this practice calls for a high degree of mass producibility and extra-high precision working. For example, approximately 1000 impressions must be worked in a minute. The depth of scribing ranges from tens of .mu.m to under 10 .mu.m, sometimes with such a close tolerance as .+-. a few .mu.m. Because these requirements are hard to fulfill with common scribing apparatus, the use of high-speed precision presses has been proposed recently.
The high-speed precision presses are equipped with an automatic control mechanism that detects the bottom dead center of the press stroke in operation and compensate for any change therein, the mechanism comprises means to change the die height by means of an AC servo motor or other type of actuator provided to a connecting rod. More specifically, the control mechanism consists essentially of a bottom dead center sensor (an eddy-current gap sensor) 52, an angle sensor 44, a servo motor 48 to correct the slide position, and a microcomputer 46 as shown in FIG. 11.
The mechanism shown in FIG. 11 operates as described below. The rotating energy supplied from a main motor 41 and stored at a flywheel 42 is converted into the vertical reciprocating motion of a slide 50 through a crankshaft 43 and a connecting rod 49. A top metal die 51 attached to the lower end of the slide and a bottom metal die 53 to the top of a bolster 54. The microcomputer 46 makes necessary calculation on the basis of a signal 55 that represents the displacement detected by the bottom dead center sensor (an eddy-current gap sensor) 52 and a signal 45 representing the angle of a crank 43 determined by the angle sensor 44. A resulting signal 47 representing the corrected displacement of the die height is sent to the servo motor 48 to correct the position of the slide 50. This type of mechanism can now work with a speed of 1,200 strokes per minute, with an allowable depth tolerance of .+-. 5 .mu.m (as disclosed in Japanese Patent Publication No. 5968 of 1983 and Japanese Provisional Patent Publication No. 96719 of 1985).
High-speed precision presses of the type just described are very expensive because they need position sensors, controls and other feedback mechanisms to obtain the desired accuracy. Furthermore, they cannot cope with the variables due to the crown and other factors induced by the strip to be worked, because the object of their bottom dead center control is to make up for the thermal displacement of the press and the inertia force and initial backlash of the reciprocating segments.
Strip crown results when the axial deflection in the rolling rools is transcribed onto the strip being rolled. With a 1000 mm wide strip, thickness is generally smaller in a 400 mm wide central area by not greater than 1 .mu.m and in edges by between 4 to 8 .mu.m. Thus, strip crown is trapezoidal in cross section. Hydraulic presses may seen to offer solution for the problems associated with the profile change of the strip. Actually, however, they are difficult to use because of speed limitations and the need to make their size large enough to hold the required hydraulic liquid.
Hydraulic presses are capable of implementing control by transforming, in advance, a deflection matched to the deformation resistance of the strip into a force known as deformation resistance. Therefore, a hydraulic press can follow strip crown or other profile changes by adjusting the load distribution on its fixed receiving die. For example, load distribution may be controlled so that the bottom die follows the crown in the strip. But ordinary change-over and other valves used in the hydraulic circuit to control the load on the movable top die that determines the depth of scribing are incapable of quick response because they commonly operate with a time lag of 0.2 to 0.3 s. Some special-purpose servo valves have as short a response time as 0.02 second. In a pressing cycle, however, the top die coming in contact with the workpiece will reach the bottom dead center in a much shorter time. Assume that a press working with a speed of 600 strokes per minute and a stroke of 15 mm scribes to a depth of 0.1 mm. Then, the top die of this press reaches the bottom dead center in only 0.0026 second after coming in contact with the workpiece. Therefore, even quick responding servo valves are useless. The uselessness becomes more pronounced when the scribing depth is smaller and the operating speed is faster. They may be used if the scribing depth is greater and the operating speed is slower. For example, the time to reach the bottom dead center after coming in contact with the workpiece will be 0.083 second when the scribing depth is 1 mm and the operating speed is 60 strokes per second. But the equipment must be large enough to hold large quantity of hydraulic liquid need to obtain a stroke of tens of millimeters. In addition, it is difficult to carry out high-precision scribing at high speed while following changes in the profile of the workpiece.